Hello..
you sound to me you have a clue about welding but i will go over a little basics.
Especially Tig process needs to be clean and for aluminum super clean, the tungsten, cup, gas even the surrounding atmosphere should all be looked at, that's just to get the right environment, next the alloys in a simple world should have to match, i used to make copies of exotic racing motorcycle tanks from scratch all from 16 gage plate and i cut from the plate the wire to weld them to over come this problem tig or gas welding.
Its why you cant really weld a cast iron engine block, the block is made from pig iron and you dont know what the mixture is, its achievable but just makes your life so bloody hard.
Try it with a 50/50 mix of argon/helium Its cleaner....... back yonder thats all they welded with thats why they call it Heliarc... Try The 2% thoriated electrodes Pure tungsten dont cut it. Ball it up on the stone slightly nature will do the rest.
Syncrowave with a AC fine tune on your rig, if you dont have one get one there cheap now.
Stand next to a microwave for 3 minutes and you will get more radiation than using thoriated electrodes, so dont worry about it.
I think you will stand a chance if you do what i suggested.
Hope this helps and let us know how you get on...
All the best. Anthony.